Fundamental aim

• enable 2 people to communicate over n insecure channel in such way that their connection is secure, as in an opponent (eavesdropper) can not understand them

Cryptosystem

• 5-tupe P,C,K,E,D
  • P = plaintext
  • C = ciphertext
  • K = possible keys
  • for each k in K there is an encryption rule e in E and a description rule d in D
• each encryption function must be injective
• if P = C then each e(k) is a permutation

Protection

• there’s no such thing as a perfectly, 100% secure, computer system, there will always be bugs and security experts know that
• system architects employ a strategy called defence in depth, which uses many layers of varying security mechanisms to frustrate attackers
• it’s a bit like how castles are designed:
  • first you’ve got to dodge the archers, then cross the moat, scale the walls, avoid the hot oil, get over the ramparts, and defeat the guards before you get to the throne room, but in this case we’re talking about one of the most common forms of computer security

Cryptography

• cryptography = ‘crypto’ and ‘graphy’, roughly translating to “secret writing”
• cipher
  • an algorithm that converts plain text into ciphertext, which is gibberish unless you have a key that lets you undo the cipher
• encryption = process of making text secret
• decryption = the reverse process

Substitution ciphers

• ciphers have been used long before computers showed up:
  • Julius Caesar used what’s now called a Caesar cipher, to encrypt private correspondence
    • he would shift the letters in a message forward by three places
      • A became D, and the word "brutus" became this: "euxwxv"
    • to decipher the message, recipients had to know both the algorithm and the number to shift by, which acted as the key
• the Caesar cipher is one example of a larger class of techniques called substitution ciphers
• replace every letter in a message with something else according to a translation
• drawback of basic substitution ciphers is that letter frequencies are preserved